Neural stem cells are multipotent cells which give rise to neurons and glia of the mammalian central nervous system. Recently, we found that differentiation of neural stem cells is altered in fragile X syndrome, a developmental brain disorder with disturbances in the molecular mechanisms that mediate learning and memory. The absence of fragile X mental retardation protein caused an increased number of new-born cells in the subventricular region of the embryonic mouse brain and substantial aberrances in the differentiation of both human and mouse neural stem cells in vitro. Here, alterations of neuronal cell differentiation in fragile X syndrome, the implications of our recent findings, and some open questions that need to be addressed, are discussed.